What's the difference between an object created in function call and object passed in

Question

This is obviously a hole in my self taught computer science education...

The constructor for a text control (wxTextCtrl) in a wxWidgets application has an optional parameter for a validator object. All of the code examples create the validator on the fly within the constructor for the text control.

This works..

wxString value = L"0.0";
wxTextCtrl* _Text = new wxTextCtrl(this, wxID_ANY, value, 
    wxDefaultPosition, wxDefaultSize, 0, 
    wxTextValidator(wxFILTER_NUMERIC, &value));

However in my particular case I want to create the validator in another function and pass it back, which isn't working. As an intermediate step I've tried to create it just before creating the wxTextCtrl and pass it in but that doesn't work either...

wxString value = L"0.0";
wxValidator valid = wxTextValidator(wxFILTER_NUMERIC, &value);
wxTextCtrl* _Text = new wxTextCtrl(this, wxID_ANY, value, 
    wxDefaultPosition, wxDefaultSize, 0, valid);

Although this compiles and runs it doesn't perform the validation. Can anyone explain why?

The prototype for the wxTextValidator calls for a constant reference..

wxTextCtrl::wxTextCtrl  (   wxWindow *  parent,
    wxWindowID  id,
    const wxString &    value = wxEmptyString,
    const wxPoint &     pos = wxDefaultPosition,
    const wxSize &  size = wxDefaultSize,
    long    style = 0,
    const wxValidator &     validator = wxDefaultValidator,
    const wxString &    name = wxTextCtrlNameStr 
)

The third parameter to your `new wxTextCtrl()` calls is different: `value` vs `_controls[name]`. Is this intentional? — Tavian Barnes, Oct 25 '16 at 16:21

Tavian Barnes · Accepted Answer · 2016-10-25T16:35:30.800

6

You have sliced the wxTextValidator object when you assigned it to a variable of type wxValidator, its base class. To fix this, you'll need to preserve the more specific type:

wxTextValidator valid = wxTextValidator(wxFILTER_NUMERIC, &value);

You can use auto to avoid repeating yourself.

Alternatively, you can use lifetime extension which occurs when assigning a temporary to a const reference:

const wxValidator& valid = wxTextValidator(wxFILTER_NUMERIC, &value);

This works because there is no copy, and therefore no slicing.

Note that when designing your own classes, it's often a good idea to prevent object slicing by making your base classes abstract, or making their (copy) constructors protected.

edited Oct 25 '16 at 16:35

answered Oct 25 '16 at 16:25

Tavian Barnes

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Fixing that could be either: `wxTextValidator valid{wxFILTER_NUMERIC, &value};` or `auto valid = wxTextValidator(wxFILTER_NUMERIC, &value);` (both of which also mean you avoid repeating yourself in ways that could accidentally get out of sync). – ShadowRanger Oct 25 '16 at 16:27
@ShadowRanger Indeed. Even `const wxValidator& valid = ...` would work due to lifetime extension. – Tavian Barnes Oct 25 '16 at 16:29
@TavianBarns I'll (obviously) have to do some more reading, but isn't that exactly what is happening in the working example as well? – marcp Oct 25 '16 at 16:32
@marcp Yes it is. – Tavian Barnes Oct 25 '16 at 16:36
@ShadowRanger, I don't understand the syntax of your first suggestion. What is that called? – marcp Oct 25 '16 at 16:43
@marcp: It's the uniform initialization syntax introduced in C++11. Pre-C++11, you'd use `wxTextValidator valid(wxFILTER_NUMERIC, &value);`, for this purpose, they're largely equivalent (some people prefer parens, some braces). [See more here](http://stackoverflow.com/q/18222926/364696). – ShadowRanger Oct 25 '16 at 16:52

What's the difference between an object created in function call and object passed in

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